Metallic foam material

ABSTRACT

Tip rub in gas turbine engines is a well-known phenomenon, and may lead to increased tip clearances, with consequent detrimental effects on the engine&#39;s performance and blade flutter margins. Heavy or repeated rubbing may cause blade tip over temperature leading to cracking and fatigue failure. The invention provides a tip for a gas turbine engine rotor blade, which under contact with the casing surface interacts with the casing surface so as to reduce the contact force and thereby reduce the energy transferred to the blade. The tip may be designed to ablate under contact with the casing surface, or to cut or abrade the casing surface, or to crush or compress of deflect under contact with the casing surface. In a preferred embodiment, the tip comprises fibre-reinforced metallic foam material.

This invention relates to gas turbine engines. More specifically, it relates to tips for rotor blades in ducted gas turbine engines.

Tip rub in gas turbine engines is a well-known phenomenon, involving the interaction of rotor blade tips with the casing surfaces surrounding them. Tip rubs may lead to increased tip clearances, with consequent detrimental effects on the engine's performance and blade flutter margins. Heavy or repeated rubbing may cause excessive heating of the blade tip, leading to cracking and fatigue failure.

A further problem with tip rubs is that periodic rubbing can promote vibration within the rotor blades. This can be a particularly serious problem if the frequency of the rubbing corresponds to a resonant frequency of the rotor blade, casing, or liner panel.

Known arrangements accept that tip rubs will occur, and aim to reduce their consequences by allowing either the blade tip or the casing liner to be worn away by a harder material on the other component. However, this inevitably leads to increased tip clearances and the detrimental effects noted above. No known designs aim to reduce the blade/liner interaction forces; such forces may lead to blade cracking or fatigue failure, or to increased vibration which in turn can cause heavier tip rubs.

In known arrangements in which the blade tip is arranged to be harder than the casing, it is known to provide a discrete tip portion on the rotor blade. This tip portion is of harder material than the casing liner, and may also be harder than the rest of the blade. The tip portion may be formed as part of the blade or may be attached to the blade in a number of ways, including by bonding.

There is a particular problem with rotor blades made from fibre-reinforced composite materials (such as Carbon Fibre Reinforced Plastic—CFRP). The heat generated by tip rubs must not be sufficient to damage the composite, or to change its operational properties (whether temporarily or permanently), in particular by damaging the matrix material or by causing deterioration of the adhesive attaching the blade tip or other components to the blade. The longer the blade/liner interaction forces persist, the more heat is likely to be generated in the tip. In known arrangements, composite rotor blades have been designed with relatively large tip clearances so that there is no (or, at worst, rare and brief) contact between the blade tips and the casing under normal operating conditions, but this reduces the efficiency of the engine because there is always flow leakage around the blade tips.

Accordingly, a first aspect of the invention provides a tip for a gas turbine engine rotor blade, characterised in that under contact with the casing surface the tip interacts with the casing surface so as to reduce the contact force and thereby reduce the energy transferred to the blade. In this way, the detrimental effects noted above, such as blade cracking, fatigue and increased vibration, are avoided or mitigated.

The blade tip may be designed to ablate, or to cut or abrade, or to crush or compress or deflect, under contact with the casing surface. These mechanisms allow the contact forces to be reduced, without excessive heating of the blade tip and while maintaining acceptable aerodynamic performance in service.

The blade tip may comprise fibre-reinforced metallic foam material. The reinforced metallic foam acts as a framework and strengthening for the blade tip, and provides damping of vibrations.

The reinforcing fibres of the tip may extend into the rotor blade. The fibres provide tensile strength, and by extending them into the rotor blade the attachment of the blade tip to the rotor blade is improved.

The reinforcing fibres of the tip may be interleaved with the reinforcing fibres of the blade. This allows a matrix or adhesive material to act both in tension (at the butt joints) and in shear (in the sliding regions between the fibres). The castellations prevent the adhesive from going into peel.

The blade tip may comprise a damping medium. This damps blade vibrations and also prevents transmitted vibrations from the tip rubs from disturbing airflow over the rest of the blade.

A second aspect of the invention provides a gas turbine engine rotor blade comprising a blade tip as described in the preceding six paragraphs.

Embodiments of the invention will now be described in more detail, with reference to the attached drawings, in which

FIGS. 1( a)-(c) show three embodiments of an ablative blade tip according to the invention;

FIGS. 2( a)-(d) show four embodiments of a cutting blade tip according to the invention;

FIGS. 3( a)-(b) shows two embodiments of a crushable cutting blade tip according to the invention; and

FIG. 4 shows an embodiment of a blade tip according to the invention and incorporating a damping medium.

FIG. 1 shows three embodiments of an ablative blade tip according to the invention. An ablative tip is one that, in use, wears or erodes away in a controlled manner.

Referring first to FIG. 1( a), the tip region 12 of a rotor blade 14 has a pressure surface 16 and a suction surface 18. The rotor blade is formed of fibre-reinforced composite material 20.

A blade tip 22 is attached to the radially outer end of the rotor blade 14. The blade tip 22 is formed of sacrificial temperature-tolerant material and comprises reinforcing fibres 26 which extend generally in a radial direction through the blade tip 22. The reinforcing fibres 26 are typically carbon fibres, and may be treated with a coating such as glass to provide galvanic isolation and resistance to spalling. Other fibres 26 such as silicon carbide (SIC), boron or aramid may be selected depending upon the operating environment and surrounding materials. A combination of different types of fibre may be used. The sacrificial material may be chosen to be hard and crumbly, such as a sintered or compressed powder or ceramic or metal (e.g. aluminium and SiC); a medium density such as a phenolic resin; or a lower-temperature-tolerant material such as a thermoplastic that will smear or “spall” under contact temperatures. There are a variety of prior art materials that will suffice.

The rotor blade 14 has facing sheets 28, 30 respectively on its pressure and suction surfaces. These form part of the protective metalwork commonly used on composite blades; the full extent of this is not shown. The blade tip 22 and the facing sheets 28, 30 are secured to the composite material 20 by layers of adhesive 32, 34. Suitable adhesives, in a preferred embodiment of the invention, would be those sold by the 3M Company under the trade marks AF32 (for layer 32) and AF500 (for layer 34). The adhesive may be a damping layer adhesive or a multilayer adhesive sandwich.

FIG. 1( b) shows a second embodiment of an ablative blade tip according to the invention. A number of elements are identical to those in FIG. 1( a), and the same reference numbers indicate these elements.

In the embodiment of FIG. 1( b), the blade tip 122 is formed of composite material comprising chopped fibres in a matrix. This may be a combination such as chopped carbon fibre in an epoxy matrix or chopped glass and carbon in a phenolic resin; the skilled reader will appreciate that other mixtures are possible. As in the embodiment of FIG. 1( a), generally radially extending carbon fibres 126 extend through the blade tip 122 to provide reinforcement and attachment.

FIG. 1( c) shows a third embodiment of an ablative blade tip according to the invention. A number of elements are identical to those in FIG. 1( a), and the same reference numbers indicate these elements.

In the embodiment of FIG. 1( c), the blade tip 222 is formed of fine-pored metallic foam, such as the nickel foam produced by Recemat International and sold under the trade mark RECEMAT. Similar foams are made by other companies from a variety of materials, from which a suitable strength and weight may be selected. As in the embodiment of FIG. 1( a), generally radially extending carbon fibres 226 extend through the blade tip 222 to provide reinforcement. These fibres can extend into the metal foam or be incorporated as part of the metal foam manufacturing process, as described in United Kingdom patent application GB1200034.5.

In FIGS. 1( a) and 1(b) a section of fibre reinforced metallic foam can replace the is sections 26, 126, where the fibres provide the link between the foam and the composite blade, and the foam provides the key to secure the ablative material. Should all the ablative material be removed it provides a robust tip to protect the composite blade. For this reason, this area is shown protruding beyond the facing sheets 28, 30 so that any contact between a worn blade tip and case is not with the facing sheets.

In use, the ablative blade tip 22, 122, 222 is designed to rub against a casing (not shown) of the gas turbine engine, and to be gradually worn away by that contact. It is envisaged that about 1.2 mm of the blade tip material would be worn away in normal service before the blade is removed for repair. Under exceptional circumstances, up to 3 mm of blade tip material may be worn away without causing irreparable damage. It is desirable to limit the wear of ablative tips, for at least two reasons. Firstly, if one blade wears much more than the others it will affect the balance of the rotating assembly. Secondly, if the tips are designed so that a lot of material will be worn away in use, the resulting tip gap will of course be large, with detrimental effects on the engine's efficiency. Also, if the facing sheets contact the casing liner they will cut the liner, and in effect will act as an abrasive tip. Typically the tips will extend 5-8 mm beyond the facing sheets. The blade tip is sufficiently robust to resist damage from typical hail and ice impacts that occur during normal service.

In arrangements using ablative blade tips as shown in FIGS. 1( a), (b) and (c), there will be no appreciable wear of the casing material in normal use. As the skilled reader will appreciate, the casing is designed (in accordance with normal practice) to form part of the engine's containment system and to be resistant to ice and debris impacts.

The fibre reinforcement may extend from the blade tip into the rotor blade 14. In this case the reinforced tip is manufactured with a set of dry fibre or prepreg tows that are inserted into the blade ply layup at the appropriate stage; the tip is then co-cured with the blade. The alternative method of leaving tows from the blade free and extending at the tip to form the reinforced tip is more complicated and is similar to the tip replacement method.

FIG. 2 shows four embodiments of a cutting or abrasive blade tip according to the invention. A cutting tip is one that, in use, cuts into a relatively soft abradable layer on the casing. In normal use, a cutting tip does not sustain appreciable damage. An abrasive tip is one that in use cuts into the casing abradable layer by abrading it using a series of hard particles—essentially a series of small cutters—or by utilising a cutting edge.

Some elements of the arrangements shown in FIG. 2 are identical to corresponding elements in FIG. 1, and the same reference numbers indicate these elements.

FIG. 2( a) shows the tip region 12 of a rotor blade 14, with a pressure surface 16 and a suction surface 18. The rotor blade is formed of fibre-reinforced composite material 20.

A blade tip 42 is attached to the rotor blade 14. The blade tip 42 is formed of abrasive temperature-tolerant material and comprises reinforcing carbon fibres 46 which extend generally in a radial direction through the blade tip 42. Facing sheets 48, 50 of the rotor blade 14 extend all the way to the tip of the blade. The pressure surface facing sheet 48 acts as the cutting edge in the conventional manner.

In this embodiment, the radially outer face of the blade tip 42 is shaped so that the edge 52 of the blade tip adjacent the pressure surface 16 of the rotor blade 14 (which, in operation, will be the “leading” edge of the blade tip as the rotor blade rotates) extends further than the edge 54 adjacent the suction surface 18 (which, in operation, will be the “trailing” edge of the blade tip). The cutback angle (the angle from the leading to the trailing edge) in FIG. 2( a) is shown as linear; it may alternatively be shaped or curved, as is known with other cutting tools. Such curvature can be advantageous, as the blade will bend and twist slightly due to the frictional force of its contact with the casing or casing liner and in response to wind gust loads.

In other embodiments of the cutting tip, such as those shown in FIGS. 2( b), (c) and (d), the blade tips have alternative profiles. In the drawings these are exaggerated for clarity. Such blade tip shapes give a number of advantages, notably: better compressive strength in the radial direction; better distribution of heat; a known cutting edge position; and enhanced cutting during heavy rubs. For a particular application, the optimum profile will be dependent upon the blade stiffness in the radial and twist directions, and the optimum angle to cut the casing liner material, which will in turn determine the optimum angle and shape for the blade tip. The tip profile may be symmetrical; or (as with cutting tools) non-symmetrical arrangements may be preferable, such as having a steeper angle of attack on the pressure surface. The optimum angles will depend upon cutting speed (blade tip speed), the material being cut, and the anticipated depth of cut.

In the embodiment of FIG. 2( b), the composite material 20 of the rotor blade 14 extends all the way to the tip of the blade. Alternatively the fibres are interleaved to allow the tip to be replaced/repaired in the future. This may be done, for example, by interleaving the fibres like courses of bricks. This allows a matrix or adhesive material to act both in tension (at the butt joints) and in shear (in the sliding regions between the fibres). The castellations prevent the adhesive from going into peel. The tip of the blade has an abrasive coating 56.

In the embodiment of FIG. 2( c), the blade tip 142 is formed of Ti 6-4 and extends within the facing sheets 148, 150 towards the rotor blade 14. The blade tip 142 comprises reinforcing carbon fibres 58 which extend generally in a radial direction through the blade tip 142. As in the embodiment of FIG. 2( b), the tip of the blade has an abrasive coating 56. The core of reinforced fibres can be interfaced to the titanium blade tip structure 142 by forming a metallic foam structure around the fibres, as in UK patent application GB1200034.5. This has the advantage that the metallic foam gives crush (compression) protection, and the fibres provide the tensile strength. As in FIG. 2( b) the fibres can be made to extend all the way to the tip.

In the embodiment of FIG. 2( d), the blade tip 242 is again formed of Ti 6-4, but surrounds the tip region of the rotor blade 14 and extends around the outer sides of the facing sheets 148, 150. As in the embodiment of FIG. 2( b), the tip of the blade has an abrasive coating 56. This embodiment also has the advantage that a damping matrix material can be added to, or can form, the adhesive that bonds the tip 242 to the rotor blade 14.

In all of the embodiments of FIG. 2, the corresponding casing liner material would be an abradable material chosen to minimise the heating of the cutting tip in operation. As in normal practice the material is chosen to be as low in density as possible to enable ease of cutting, but of high enough density to resist ice and FOD impacts. A particularly suitable arrangement, known in the prior art, is to use a low-density abradable material within a framework (such as a honeycomb of card (Nomex™) or aluminium) to support the abradable and prevent it being pulled off in chunks by the cutting tip.

FIG. 3 shows two embodiments of a crushable cutting blade tip according to the invention.

Some elements of the arrangements shown in FIG. 3 are identical to corresponding elements in FIG. 1, and the same reference numbers indicate these elements.

In FIG. 3( a), a rotor blade 14 has a blade tip 64 of coarse-pored nickel foam, such as that produced by Recemat International and sold under the trade mark RECEMAT. The blade tip also has a fibre reinforced core similar to that described in GB1200034.5.

In FIG. 3( b), the rotor blade 14 has a blade tip 164 of cellular titanium with anisotropic properties. This anisotropy is achieved by creating the foam not with regular dodecahedral cells but with one axis of the cells longer or shorter than the other axes. This is preferably done during the manufacturing process, such as in GB1200034.5 where oval rather than spherical formers are used. The anisotropy can be achieved by stretching or rolling in one axis—though care must be taken not to damage the structure during these processes.

Embedded in the blade tips 64, 164 are reinforcing fibres 66, 166. In both embodiments, the fibre-reinforced metal foam is bonded directly on to the metal surface of the blade 14 and an abrasive layer 68, 168 (such as cubic boron nitride or zirconia oxide) is embedded into the metal foam top surface. In operation, the abrasive layer 68, 168 will remove part of the abradable material of the casing liner (not shown). The casing liner may be made of a single layer of abradable material, or may mirror the blade tip design to increase the benefit of the metallic foam especially if the matrix material has damping properties (an example is shown in FIG. 4). The reinforced metallic foam acts as a framework and strengthening for the abrasive material, and as damping for any contact vibration.

In a conventional blade, during a normal blade tip liner interaction, the only energy dissipation mechanism is associated with the removal of the material from the blade tip or the liner. The contact energy is therefore transferred almost entirely to the blade, which is converted to vibration energy that may lead to subsequent heavier rubs. In contrast, the blade tip rub design here acts differently: as the blade tip liner interacts, the blade tip will absorb the contact energy by compressing (deflecting) more, being more compliant than a conventional blade tip. This reduces the contact force and so reduces the energy transferred to the blade and the amount of removed abradable due to blade instabilities.

Following the blade and case contact interaction, the blade has lower vibration energy and so will keep tighter clearance (from blade tip to casing liner). A further advantage is that the resultant lower contact forces and contact time will lead to lower tip temperatures so reduced likelihood of tip cracking or thermal distortion. The reinforced foam layer acts as a thermal barrier for the bulk of the blade. This is particularly valuable if the blade is composite, where the composite matrix material or the adhesive securing the facing sheets 28, 30 may soften or distort at higher temperatures.

In any of the embodiments so far described (FIGS. 1 to 3) a damping medium may be included within the blade tip. This damps the blade vibration from the changing tip path (as described above) and also prevents transmitted vibrations from the tip rubs from disturbing airflow over the rest of the blade. FIG. 4 shows schematically how a damping medium 70 may be incorporated into a blade tip according to the invention. Suitable damping materials would preferably be incorporated within the matrix, infused into the foam and around the fibres. The most common way to do this is to incorporate elastomeric particles into the resin matrix. This can be enhanced by using an elastomeric adhesive layer between the core and the sheaths (e.g. item 34 in FIG. 1) and the abrasive/abradable tip.

The invention therefore provides an improved design of rotor blade tip, which accommodates the forces and heat transfer associated with tip rubs and is therefore particularly suitable for use in composite rotor blades. The key advantage of the invention is that it allows gas turbine engines, even those with composite blades, to run with tight tip clearances, thereby improving the engines' overall efficiency.

It will be appreciated that the embodiments described represent only examples of ways in which the invention can be put into effect, and are not to be regarded as limiting. 

1. A fibre-reinforced blade tip for a gas turbine engine rotor blade, the tip designed to contact in use a casing surface of the gas turbine engine, characterised in that under contact with the casing surface the tip interacts with the casing surface so as to reduce the contact force and thereby reduce the energy transferred to the blade.
 2. A blade tip as claimed in claim 1, in which the tip is designed to ablate under contact with the casing surface.
 3. A blade tip as claimed in claim 1, in which the tip is designed to cut or abrade the casing surface under contact.
 4. A blade tip as claimed in claim 1, in which the tip is designed to crush or compress or deflect under contact with the casing surface.
 5. A blade tip as claimed in claim 1, in which the tip comprises fibre-reinforced metallic foam material.
 6. A blade tip as claimed in claim 5, in which at least some of the reinforcing fibres of the tip extend into the rotor blade.
 7. A blade tip as claimed in claim 5, in which the rotor blade comprises fibre-reinforced material and in use the reinforcing fibres of the tip are interleaved with the reinforcing fibres of the blade.
 8. A blade tip as claimed in claim 1, and further comprising a damping medium.
 9. A gas turbine engine rotor blade comprising a blade tip as claimed in claim
 1. 